Dynamic Range is an important metric in an integrated circuit or a system having a digital to analog signal path. One example of such a circuit or system is a digital audio playback device, such as an mp3 player, CD player, or mobile phone. Such playback devices have means, described below, for transforming digitally stored or transmitted data into a usable analog signal. Dynamic range (DR) is the ratio between the smallest and largest possible values of a changeable quantity, such as audio signal amplitude. DR is typically measured as a ratio, such as a base-10 (decibel) or base-2 (doublings, bits or stops) logarithmic value. In most electrical systems, including digital audio playback, the decibel measure is standard. For example, the 16-bit compact disc has a theoretical dynamic range of about 96 dB. Digital audio with 20-bit digitization is capable of approximately 120 dB DR; similarly, 24-bit digital audio calculates to approximately 144 dB DR. In audio playback systems, it is common practice to measure DR as total harmonic distortion plus noise (THD+N) relative to full scale with a −60 dB input signal. Furthermore, the approximate theoretical maximum DR values are sometimes calculated relative to a full Nyquist frequency bandwidth measurement. In some measurement techniques, measured bandwidths are reduced from the Nyquist rate to an audible bandwidth resulting in an increase in the reported dynamic range value. Also, the frequency spectrum of the measured output can be altered using industry accepted filters that may alter the DR value. For example, some measurement techniques filter an output through an A-weighted filter that approximates the perceptive response of the human ear.
Conventionally, digital audio recording and playback chains include input and output converters and associated analog circuitry, which may result in significantly limiting practical DR below the theoretical optimum. Observed 16-bit digital audio DR can be noticeably lower, even 90 dB or below, when multiple circuits in the audio path individually contribute integrated noise at the −96 dB level. A DR measurement is a useful indication of signal quality when the desired signal level is low. In an audio example, soft music or low volume music can suffer from the effects of poor DR more than loud music.
FIG. 1 shows such a prior art digital to analog signal path 100 that may have limited DR performance. Digital input data 110 is coupled to a digital gain block 120. Digital input data 110 can be a digital transmission received by an antenna, stored digital data in a memory, or any other digital input source. It is important to note that the digital source need not be limited to audio data, but can be any type of quantized or digital information. In practice, the digital input data 110 is modeled as a combination of the digital input data 110 and the inherent quantization noise 112 due to the quantized nature of the signal. In FIG. 1, the quantization noise 112 is shown summed with the digital input data 110 but it is important to note that no physical summer 115 exists. Rather, the summer 115 merely represents the addition of unavoidable quantization noise 112. The digital gain 120 controls the gain of the overall signal while it is still in a digital state. Typical signal sources for providing digital input data 110 can be via a de-serialized Inter-IC Sound (I2S) bus, SPDIF de-framed signal source, or time domain recovered signal from a compressed MP3 formatted source. The source data is typically interpolated to improve the transient signal quality. The digital gain 120 controls the gain of the overall signal while it is still in a digital state, typically through manipulation using a common digital multiplier. Excessive digital gain can cause overflow in the digital code, which would typically be clip limited, and will undesirably degrade large amplitude signals.
The gained or attenuated digital signal from digital gain block 120 is then passed to a digital to analog (DAC) 130 for conversion to an analog signal. Every DAC effectively injects additional noise into the signal, which can be input referred or output referred depending on convenience and convention. In FIG. 1, DAC input referred noise 122 is shown summed with the output of the digital gain 120. Again, no physical summer 125 exists but rather serves to represent that noise is being injected into the signal path. The output of the DAC 130 is coupled with a driver and analog gain 140, which also has some input referred noise 132. The summer 135 merely represents that even more noise is being injected into the system. It is important to note that not all prior art includes both a digital gain 120 and analog gain control 140. Some systems may include simply one or the other, or neither. It is further important to note that the signal path 100 is highly simplified, and many other components as required by specific applications can be placed along the signal path 100. For example, analog mixers are often placed between the DAC 130 and the driver and analog gain 140, and each additional component will necessarily add noise to the system. The overall gain of the signal path is controlled by a gain control 150, which may control both the digital gain 120 and the analog gain 140. The gain control 150 is controlled in turn by a user control 152. An important distinction should be made between the user of the integrated circuit and the user of an end product. The user of the integrated circuit in which this signal path can be found (for example, an audio codec or processor) is most likely a manufacturer of audio appliances, such as Apple® in making their widely popular iPod®, or any other conceivable manufacturer. Such a user designs a system in which an end user, or the user of the iPod®, for example, is able to manipulate volume as desired. In general, the manufacturer's system enables the end user to manipulate one or both of the gains with a single control input. Generally, an end user cannot manipulate the digital gain 120 and the driver and analog gain 140 separately, because end users prefer a single gain control for simplicity and convenience. When an end user manipulates the volume control on their audio device, the gain control 150 as designed by the manufacturer simply adds gain in the digital realm with the digital gain 120 and/or the analog realm with the audio gain 140. However, such a system does nothing to improve dynamic range, especially in low level signals. In the prior art solutions the noise contributions of the multiple modules in the signal path require better noise performance from each of the modules. However, greater quality and lower noise modules require greater die size and more current. Die size is the greatest component of cost in an integrated circuit, and increased current is undesirable to manufacturers of portable electronics who are concerned with battery life. Furthermore, the integrated circuit industry as a whole is extremely cost driven, and increased cost associated with larger die sizes is undesirable.